New cation-deficient vanadium–iron spinels with a high vacancy content

doi:10.1016/S0025-5408(99)00165-8

Materials Research Bulletin

Volume 34, Issues 10–11, July–August 1999, Pages 1735-1747

https://doi.org/10.1016/S0025-5408(99)00165-8 Get rights and content

Abstract

New cation-deficient vanadium–iron spinels V_8x/(8+δ)Fe_{(24−8x)/(8+δ)}□_3δ/(8+δ)O₄ have been obtained by the oxidation at low temperature (<500°C) of the corresponding nanosized stoichiometric spinels V_xFe_3−xO₄ (0 ≤ x ≤ 2). The oxidation state of the cations and their distribution over both octahedral (B) and tetrahedral (A) sites were partially investigated by associating thermal analyses (differential scanning calorimetry (DSC) and differential thermogravimetric (DTG)), X-ray diffraction, and infrared and X-ray photoelectron spectroscopy. The high number of vacancies results of the Fe²⁺ to Fe³⁺ and V³⁺ to V⁵⁺ through V⁴⁺ ions oxidation. This one may reach 0.90 per unit formula for x = 2.

Introduction

We showed in previous publications that the oxidation of substituted magnetites M_xFe_3−xO₄ (0 ≤ x ≤ 1) may lead to the formation of cation-deficient phases (typically γ-Fe₂O₃) if the initial powders have nanometric sizes. Actually, these cation-deficient spinels result from either the only oxidation of Fe²⁺ cations if the Mⁿ⁺ cations are in a stable valency state, such as Cr³⁺, Al³⁺, Co²⁺, or Zn²⁺ 1, 2, 3, 4, or the oxidation of ferrous cations and Mⁿ⁺ cations as Mn²⁺ and Mo³⁺ 5, 6 if they can present one or several higher valency states.

Up to now, elaboration of spinels containing iron and vanadium consists of a classical ceramic route, which leads to grains having sizes of about 2 μm. This large grain size does not allow the total oxidation of the spinel while retaining the same crystallographic structure. This paper demonstrates the ability of V_xFe_3−xO₄ spinels (0 ≤ x ≤ 2) to give, from iron and vanadium ion oxidation, cation-deficient spinels having V⁵⁺ ions and high vacancy content.

Section snippets

Experimental

Stoichiometric vanadium–iron spinels V_xFe_3−xO₄ were prepared by thermal treatments initially under air at 500°C and then under H₂–N₂–H₂O mixture at the same temperature [7]. Precursors were mixed oxide particles obtained by precipitation of Fe²⁺, Fe³⁺, and V³⁺ chloride solutions in alkaline medium. The elaborated stoichiometric phases were finely grained spinels, whose crystallite size was close to 50 nm. The structural formula (Fe_α²⁺Fe_1−α³⁺)_A(Fe_1−α²⁺Fe_1−x+α³⁺V_x³⁺)_BO₄²⁻, which is generally

Oxidation behavior

In Figure 1 we show the thermogravimetric (TG) curves of V_xFe_3−xO₄ spinels. The mass gain increases with the vanadium content and is always higher than the mass gain measured for the total oxidation of Fe₃O₄ (3.44%). Therefore, when x > 0, the reaction was due to not only iron ion oxidation, but also vanadium ion oxidation. Experimental and calculated values for total oxidation above 500°C are close enough to assume that the oxidation of Fe²⁺ to Fe³⁺ and V³⁺ to V⁵⁺ cations occurs.

Similar to

Conclusions

These results show that vanadium–iron spinels V_xFe_3−xO₄ with 0 ≤ x ≤ 2 prepared at low temperature and having a crystallite size close to 50 nm are highly reactive with oxygen. Consequently, they could be totally or partly oxidized below 500°C to form cation-deficient spinels with a larger vacancy content than that of the parent γ-Fe₂O₃.

Although getting structural information from amorphous or simply poorly crystallized compounds is not easy, it was found that for x < 1, the vacancies and V⁵⁺

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Original ArticlesNew cation-deficient vanadium–iron spinels with a high vacancy content

Abstract

Introduction

Section snippets

Experimental

Oxidation behavior

Conclusions

J. Solid State Chem.

Mater. Res. Bull.

Solid State Ionics

J. Phys. Chem. Solids

J. Solid State Chem.

C.R. Acad. Sci. Paris, Ser. II

Thermochim. Acta

J. Solid State Chem.

Vib. Spectrosc.

Thermochim. Acta

J. Less-Common Met.

Original Articles
New cation-deficient vanadium–iron spinels with a high vacancy content